What color in particular keeps turning black? The liquid phase?

I mean, charcoal is a vast mixture of large carbon species and probably you solubilized some of them by your process. If the mixture is that broad, your solution (or maybe dispersion as well) will probably have something in it to absorb every wavelength whatsoever, so the "blackness" doesn't sound that surprising to me.

Additionally, charcoal may contain some inorganic impurities that contribute further to the absorbance.

The Hummers method for producing graphene oxide is based on oxidation and exfoliation of graphite. As far as I understand, the prerequisite for this method to work is that the precursors need to possess layered structure or at least a certain degree of layered structure. If your charcoal is composed primarily of amorphous carbon, the Hummers method might not work well with your system and hence you only observe black products.

You can perform Raman spectroscopy (probe the peak intensity of G peak to D peak) to reveal the graphitization degree of your charcoal.

Dear Ana,

Great question! I believe it would be exceptionally challenging to design such an experiment:

Article Efficient and Large Scale Synthesis of Graphene from Coal an...

While I lack to see much value behind it, due to the fact that graphite is already very inexpensive and chemically structured (as also mentioned by Tianyu) and graphene production is already a very costly process, regardless of its sensitivity and regardless of defective structures of coal, this paper may be an interesting one to look into:

Article Graphene Sheets from Graphitized Anthracite Coal: Preparatio...

Best,

J.

graphite is crystal phase of carbon while coal char is amorphous, i don't think it can be convert to GO as graphite. its better to convert it to activated carbon for your desire composite